1. Field of the Invention
The present invention relates to a photo-curable type liquid solder resist ink composition which can form a permanent protective film excellent in adhesiveness, heat resistance, chemical resistance-and resolution on a printed circuit board.
2. Description of the Related Art
Heretofore, in solder resists, there has been used mainly a method in which a thermo-curable or UV-curable type of solder resist is printed by screen printing to form patterns. However, the screen printing has a limitation in ability to form fine patterns, and therefore, can no longer keep abreast with the recent trend of fine patterning by higher integration in ultra LSI. As the result, the resist-forming method has been replaced with a method in which a photo-imaging is utilized with a tendency to increasing high density and fineness of printed substrates.
As a photo-solder resist utilizing the photo-imaging, a dry film type had been initially used. Such photo-solder resistant, however, tends to produce bubbles during compression bonding to the substrates. Therefore, nowadays, a liquid solder resist having no limitation in coating method has been highlighted.
In a pattern forming method using a photo-imaging liquid solder resist, the following serial process is used:
1. At first, applying a resist on a printed circuit board, followed by forming a film by heat-drying; PA1 2. Laminating a patterning film to the coated film by applying pressure; PA1 3. Exposing light to the resulting film to cure the exposed portion; and PA1 4. Finally, removing the non-exposed portion of the film by developing. PA1 (a) A method in which at first the novolak-type epoxy resin (I) is reacted with (meth)acrylic acid (II) [incorporated with the polyfunctional (meth)acrylate, as required], and then the reactant is further reacted with the chain-extension agent (III); PA1 (b) A method in which the novolak-type epoxy resin (I), (meth)acrylic acid [incorporated with the polyfunctional (meth)acrylate, as required] and the chain-extension agent (III) are reacted simultaneously; and PA1 (c) A method in which at first the novolak-type epoxy resin (I) is reacted with the chain-extension agent (III), and then the reactant is further reacted with (meth)acrylic acid (II) [incorporated with the polyfunctional (meth)acrylate, as required].
In this process, if tackiness are present in the coated film after heat-drying in said step 1, the pattern film can not be released or, if a pattern film can be produced, a precise pattern can not be reproduced because of sticking of a part of the resist to the pattern film. For these reasons, a tack free characteristic of a coated film present after heat-drying during pattern formation is an important required property.
In addition, other properties required for a solder resist ink are solder heat resistance, chemical resistance, adhesiveness and resolution of the coated film. In order to improve such properties, the cross-linking density of the solder resist present after photo-setting should be raised. Consequently, the photo-sensitivity of the photo-polymerizable resin which is a main component of an ink composition before photo-polymerization should be increased.
However, simply increasing the photo-polymerizable double bonds causes fragility and a decrease of adhesiveness of the resist film after photo-curing. In addition, this further causes a decrease in tack free characteristic.
Accordingly, it is necessary to design the polymer molecule of the photo-polymerizable resin so that the molecular weight of a photo-polymerizable resin is increased, and as many photo-polymerizable double bonds as possilbe are introduced into the molecule considering to the molecular weight.
In the actual molecular design mentioned above, in case where the photo-polymerizable resin is epoxy acrylate, the introduction of the photo-polymerizable double bonds can be carried out by utilizing the reaction of the epoxy group. However, even if using a novolak-type epoxy resin which is most functional and has a larger molecular weight than aliphatic epoxy compounds, the photo-polymerizable double bonds able to be introduced in one molecule is about 10, in which both molecular weight and the number of the double bonds are insufficient.